1. Field of the Invention
The invention generally relates to a gas meter, and in particular relates to a microprocessor-based gas meter having gas flow metering and safety shutoff mechanisms.
2. Related Art
Some places on the earth located near the seismic zones, such as Taiwan and Japan located on the circum-pacific seismic zone, suffer from frequent earthquakes than other places. Earthquakes not only damage building and hurt people, but also destroy facility lines, such as electric and gas supplies and cause secondary disasters.
To prevent gas leakage damages in earthquakes, Japan government acted to apply microprocessor-based gas meters from 1988. On Jan. 17, 1995, Hanshin-Awaji earthquake in Japan, with magnitude of 7.2, greatly damaged gas lines and gas facilities, caused gas leakage, fire and heavy casualties. Afterwards, Japan government actively introduces automatic gas shutoff devices in order to shut down gas lines and prevent gas leakage and fires during accidents of earthquake or other emergencies.
In the Energy Policy White Paper published by Ministry of Economic Affairs of R.O.C. in 1998, researches on microprocessor-based gas meter are suggested. Therefore, the Energy Commission and the Industrial Technology Research Institute started developments of microprocessor-based gas meter in 1999. On early morning of Sep. 21, 1999, Taiwan encountered a strong earthquake with magnitude of 7.3. The government therefore further pushes developments of automatic shutoff devices for gas facilities. The Bureau of Standards, Metrology and Inspection also plans to include inspection regulations to these kinds of devices for the sake of public and consumer's safety.
FIG. 1 shows a side view of a microprocessor-based gas meter disclosed by Japan Patent No. JP2000-9511. FIG. 2 is a top view of the gas meter. The microprocessor-based gas meter has an add-on control circuit module 20 mounted on top of a measuring chamber 10 of a conventional gas meter. The control circuit module 20 includes a cutoff valve 21 and a pressure detector 22. When an emergency occurs, the cutoff valve 21 will prevent the gas leakage.
However, the microprocessor-based gas meter separating the control circuit module 20 from the measuring chamber 10 increases the dimensions and tooling cost of the gas meter. Furthermore, the control circuit module 20 includes all the control elements on a single circuit board that is easy to generate heat and cause damage to the circuit.
Further, the microprocessor-based gas meter is not fully sealed that water or rain may get into the chamber. Because the control circuit module 20 is horizontally placed, it is easy to accumulate water on the circuit board and cause shortage of the circuit. Moreover, the control circuit module 20 is an additive member to a convention gas meter; it increases the total dimensions of the gas meter and does not better utilize the chamber space.
Then, Japan Patent No. JP2000-9511 discloses a microprocessor-based gas meter having means to overcome the drawbacks of water accumulation and circuit shortage of the horizontal circuit board on top of the measuring chamber. The microprocessor-based gas meter uses water resistive material, such as epoxy resin, silicon resin or the like, to cover the circuit board and prevent it from circuit shortage when water drops fall thereon.
However, in the microprocessor-based gas meter, the gas entering into four chambers in the lower portion first passes through a step motor that will increase resistance to the gas flow and influence the gas metering. Also, the microprocessor-based gas meter uses a solenoid to cut off the gas. Though the solenoid uses less electricity, the spring in the solenoid tends to change its elasticity after being used for a long time. It will fail to cut off the gas safely and reliably.
From an overview of the aforesaid conventional microprocessor-based gas meters, it is a common weak point that they use additive control modules on original gas meters. The add-on modules increase the dimensions of the whole units and do not fully utilize the interior spaces of the gas meters.